The most common type of varistor the metal-oxide varistor (MOV). Conventionally, a MOV contains a ceramic mass of zinc oxide (ZnO) grains in a matrix of other inorganic oxides such as small amounts of bismuth, cobalt, chromium and/or manganese sandwiched between two metal plates which act as the electrodes. The other inorganic oxides to be added to the ZnO generally have a lower melting point than ZnO, and act as a sintering additive during the sintering step when the MOV is formed.
Further, the presence of the other inorganic oxide additives is also known to affect the non-linear current-voltage characteristics of the varistor. More specifically, the type and combination of the other inorganic oxide additives used affect not only the non-linear current-voltage characteristics of the varistor, but also the sintering temperature and sintering time required to form the varistor.
Following the sintering of a conventional varistor, the varistor comprises ZnO crystal grains having a diameter of between about 10 μm to about 150 μm encapsulated by a grain boundary layer substantially consisting of other inorganic oxide additives. The non-linear electrical behavior occurs at the boundary of each ZnO crystal grain. Without being bound by theory, it is believed that the non-linear current-voltage characteristics of a varistor are dependent on the type and thickness of this grain boundary layer.
Table 1 shows how various other inorganic oxide additives affect the non-linear resistance and the non-linear coefficient (α). The larger the value of α, the higher the non-linear resistance property of the varistor. As shown in the table, if there is only one other inorganic oxide additive, the value of α is small, but when a combination of five inorganic oxide additives are used, α=50. As such, there have traditionally been many variables that can affect the properties of varistors, making it difficult to obtain varistors that have a consistent quality in performance. In addition, as the components of the varistor are ceramic, the method of forming a conventional varistor has traditionally been very complex, requiring, for example, sintering of the product at very high (approximately 1000° C.) temperatures, which often lead to instability in the varistor performance due to formation of pores. Furthermore, due to the complex process, it is difficult to maintain crystal grains of uniform size.
TABLE 1Electrical properties and average grain size of ZnOceramics with various combined additivesSinteringNon-linearAverageAdditivetemperatureresistanceNonlineargrain size(mol %)(° C.)(V/mm)exponent, α(μm)Bi2O3 (0.5)1150104.020Sb2O3 (0.5)1150653.13Bi2O3 (0.5)1250653.125CoO (0.5)Bi2O3 (0.5)1350501830MnO (0.5)Bi2O3 (0.5)1350501830CoO (0.5)MnO (0.5)Bi2O3 (0.5)1250482120CoO (0.5)MnO (0.5)Cr2O3 (0.5)Bi2O3 (0.5)13501355010CoO (0.5)MnO (0.5)Cr2O3 (0.5)Sb2O3 (0.5)
There is therefore a need to provide a varistor which overcomes, or at least partially ameliorates, one or more of the disadvantages described above.